The frequency of infection associated with the use of invasive medical devices such as insertable as well as implantable devices is well documented. In the case of insertable devices such as catheters, the rate of infection necessitates frequent replacement. In the case of implantable devices such as prosthetic devices, infections interfere with adaptation to the device. In either case, life-threatening septicemia can result from such infections.
The pathophysiology of medical device related infections is complex. Many factors influence the risk and type of infection. These include factors related to the host, to the medical device and to the virulence and inoculum of the infecting organism. Hundreds of medical publications have investigated and documented the variables that contribute to these factors. It has been well established that the overwhelming majority of medical device associated infections occur when bacteria colonize and then migrate along the medical device until they gain access to the bloodstream. Accordingly, the ability bacteria to adhere to a medical device is important to the successful establishment of an infection.
The role of bacterial surface polysaccharides in adherence is well established. Over 12 years ago a series experiments demonstrated the ubiquitous nature of these polysaccharides. Surface polysaccharides are found on most bacteria and fungi. When confronted with a specific lectin, the surface polysaccharides generate a glycoclyx that surrounds the bacteria and adhering surface. The glycoclyx consists of a mass of long polysaccharide fibers and appears to have several functions. It may act as a source of nutrition for the bacteria. It may serve as a physical barrier. Most importantly, surface polysaccharides determine the specific surface interactions of the bacterial cell.
This phenomena has far reaching effects. For example the ability of Streptococcus mutans to colonize teeth, Streptococcus salivarius to colonize gums, Bacteroides fragilus to colonize the intestine, and Group A streptococci to colonize the throat and skin are all manifestations of a complex interaction between specific surface polysaccharides and specific lectins, which are proteins that bind to specific polysaccharides.
The importance of bacterial surface and medical device related infections is best illustrated by coagulase negative staphylococci. S. epidermidis, the most important and common of the coagulase negative staphylococci, was previously considered a non-pathogenic organism. It has now emerged as the most common cause of foreign body infection and nosocomiai sepsis. It is the major cause of prosthetic valve endocarditis, vascular graft infection, artificial hip and knee infection, and catheter related sepsis. Although less virulent than S. aureus and many other bacteria, it is highly resistant to most antimicrobials except vancomycin and rifampin.
In the early 1980's, electron microscopy studies demonstrated that certain strains of S. epidermidis produce an extracellular slime like substance. The extracellular slime is a complex substance composed mostly of polysaccharide.
The production of a surface polysaccharide biofilm or slime by an organism enables it to adhere to surfaces of insertable or implantable devices and cause infection. The slime appears to contain a galactose rich polysaccharide "adhesive" which mediates attachment of the organism to polymers. It also contains a polysaccharide substance that accumulates after adherence occurs and cements the organism to the medical device.
Besides adhesion, the slime appears to have other functions. It binds to glycopeptide antibiotics including vancomycin. This may explain why most S. epidermidis infections do not respond to antimicrobial therapy alone. When infection occurs on an inserted or implanted device, removal of the device is usually required. Slime also interferes with certain immune responses.
The extracellular slime of S. epidermidis is really a manifestation of exuberant production of surface polysaccharide. The quantitative production appears to be regulated by a complex mechanism that turns on and off production based upon the local environment. Although S. epidermidis has been the focus of much of the research on foreign-body infections, this concept has been studied in other organisms. Colonization by pseudomonas species on the interior surface of PVC and other pipes has demonstrated a glycocalyx mass that shields organisms from disinfectants including chlorine, phenolics, quaternary-ammonium, and iodophor disinfectants. Once a bacterial glycocalyx is formed, it is very difficult to break down.
The development of polymers that contain antimicrobial properties has important implications for both medicine and industry. Aside from factors related to bacterial polysaccharides, the coating of the foreign body by proteins (albumin, fibronectin, platelets) from the host, as well as a variety of factors related to the polymer itself undoubtedly affect the risk of infection.
Several approaches have been utilized to produce medical devices made of or with polymers with antimicrobial properties, as described, for example, in U.S. Pat. Nos. 4,769,013, 4,713,402 and 4,886,505. Antimicrobial agents can be incorporated during the production process or grafted into the surface as described in U.S. Pat. No. 4,925,668. However, even broad-spectrum antibiotics eventually lead to the selection of resistant organisms. Selection of opportunistic fungi, resistant gram negative rods, S. epidermidis, and enterococci is likely. In addition, unless the "delivery" of the antibiotic is rapid, potent, and long lasting, formation of the protective glycocalyx will prevent its effectiveness. In addition, many antibiotics produce allergic reactions in some patients.
The present invention is based on an alternative approach, namely interference with the adherence of bacteria to polymeric surfaces of medical devices. Studies have demonstrated that both the degree of slime and adhesive production influence and correlate with the degree of bacterial adherence to silastic catheters. S. haemolyticus, unlike S. epidermidis do not produce slime and are a very uncommon cause of catheter related infection. As described herein, substances that prevent or reduce the production of slime by bacteria reduce their adherence and thus reduce the level of growth of microorganisms on the surface of the inserted or implanted devices.
It has been observed that sodium salicylates and certain other compounds can interfere with the production of capsule polysaccharide production in Klebsiella pneumonia. Salicylate binds to lipids in the outer membrane where biosynthetic enzymes are located. It has been postulated that capsular polysaccharide is the backbone of glycocalyx formation.
An object of the present invention is to use salicylates and other nonsteroidal anti-inflammatory drugs ("NSAID"), as well as other compounds such as chelating agents, to prevent the production of slime or surface polysaccharides in target microorganisms, thereby preventing their adherence and growth on materials used in medical devices.
A further object of the present invention is to utilize slime or surface-polysaccharide-inhibiting compounds which have, in addition, anti-platelet and thrombotic properties. This is particularly useful since the formation of the glycocalyx may be determined in part by platelets and fibronectin. The use of such compounds may decrease the incidence of thrombophlebitis as well as infection.
It is a further objective of the present invention to reduce bacterial growth on implanted devices using compounds that are relatively non-toxic.
These and other objectives are accomplished by the invention described in detail below.